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Fossils offer new candidate for earliest life

Fossils offer new candidate for earliest life

Scientists have uncovered tiny, iron-rich fossils that come from the depths of an ancient ocean. They were unearthed in Quebec, Canada, and date to somewhere between 4.28 billion and 3.77 billion years ago. Back then, Earth was still a baby.

These tiny structures appear to be remnants of microbes that once lived around hydrothermal vents. These are structures that spew scalding hot water from the seafloor. The tiny tube-like vents might turn out to be ancient cradles of life.

“In a nutshell, what we’ve found are the oldest microfossils on Earth,” says Matthew Dodd. He’s a biogeochemist — someone who studies the chemicals in ancient rock that may point to life. Dodd works in England at University College London.

He and his colleagues described the fossils March 1 in Nature.

But not everyone is ready to accept their conclusion. Martin Van Kranendonk is one of the doubters. He’s an astrobiologist at the University of New South Wales in Sydney. If Dodd’s structures truly are remnants of microbes, “it’s fantastic. I love it,” he says. But, he adds, “There’s just not definitive proof that any of the textures or the minerals or features they have is unique of life.”

Why identifying early life is challenging

Claims of early life often provoke controversy. Dodd admits that his team’s assessment may be hard to accept: “These are big claims — these are our origins.” Contributing to the problem is that he and others who hunt signs of early life usually have little to work with.

It’s not like they’re looking at big dinosaur bones. The early organisms were microbes — really, really tiny. What’s more, many familiar hallmarks of life may have been stripped away from the remains of billions-of-years-old microbes. And then there’s the extreme heat and pressure to which Earth’s oldest rocks would have subjected over the eons. These could have cooked and squashed any remnants of life beyond recognition.

So if scientists can’t find molded impressions of some long-gone organism, what do they use? Clues. They perform chemical tests, for instance. They analyze rock patterns and textures, some of which may be truly microscopic. Afterward, they put the clues together, hoping to amass enough evidence to determine whether the rocks point to ancient life — or not.

Last year, Van Kranendonk and his coworkers reported evidence of ancient microbial structures. They came from 3.7-billion-year-old rocks in Greenland.

Those structures were stromatolites (Strow-MAT-uh-lytes), microfossils that came from an entirely different environment. Although possibly similar in age to Dodd’s, these fossils came from shallow seas. And unlike the deep sea vents, the stromatolites might have been touched by sunlight.

So microbes may have been thriving in two very different environments, both quite early in Earth’s history. And that would mean that life arose and evolved quickly, Dodd says. It might mean that “life didn’t really struggle to get a foothold.”

What the latest study found

A filament and clump of iron ore (in lower right) may be remnants of a strand of microbial cells that are attached to rocks around the openings of hydrothermal vents.

M. Dodd

Dodd's team looked at rocks made of jasper. These seem to be leftovers of matter that long ago belched out of hydrothermal vents and then rained down on the seafloor. Dodd’s team spotted narrow tubes and filaments of an iron-rich mineral, known as hematite (HE-muh-tyte). Those filaments resemble ones that today’s bacteria make, Dodd says. Specks of iron clung to the sticky surface of the ancient microbes, he says. This would have formed a kind of armored coat that preserved the organisms’ shapes.

Other clues of early life also turned up. These included carbon minerals, such as apatite, that had earlier been linked to biological activity.

Taken together, all of those hints seemed to add up. Aside from life, he argues, “There’s no other mechanism that can explain all of these observations.”

Biology is indeed one possible explanation, says Abigail Allwood. She’s an astrobiologist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “But the evidence could equally be interpreted as non-biological.”

Each line of evidence, she points out, reflects processes that could have actually occurred at different times. When taken together, those clues may seem to add up the remains of a living organism — even if that’s not what caused it. “You can’t just wave your arms and say this all happened together,” she argues.

Paleobiologist David Wacey works at the University of Western Australia in Crawley. He agrees that “the individual lines of chemical evidence are not particularly strong.” But combined with the microstructures, he thinks that Dodd’s team offers a “pretty convincing biological scenario.”

Wacey expects the Quebec rocks to now be pored over in more detail. “There will, no doubt, be arguments,” he says. But the study may revitalize the case for hydrothermal vents as a potential birthplace for life.

Power Words

apatite A type of mineral, containing calcium and phosphate, which exists in some rocks and some types of fossils. It also is a component of bones, fish scales and the hard white enamel of teeth.

astrobiology The study of life everywhere in the universe, including on Earth and in space. People who work in this field are known as astrobiologists.

biogeochemistry A term that covers processes that cycle (or eventually deposit) pure elements or chemical compounds (including minerals) between living species and nonliving parts (such as rock or soil or water) within an ecosystem. A scientist who works in this field is a biogeochemist.

biology The study of living things. The scientists who study them are known as biologists.

carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom.

colleague Someone who works with another; a co-worker or team member.

dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs.

environment The sum of all of the things that exist around some organism or some device and the condition those things create for that organism or device.

filament Something with a thin, thread-like shape.

fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. It is technically part of North America (sitting just east of Northern Canada), and its ice sheet is the world’s largest.

hydrothermal vent An opening at the bottom of the ocean or a lake where hot water emerges from deep inside Earth. The water is rich in minerals and chemicals that can nourish ecosystems of worms, clams, microbes and other organisms.

iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.

mechanism The steps or process by which something happens or “works.”

microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microscopic An adjective for things too small to be seen by the unaided eye. It takes a microscope to view such tiny objects, such as bacteria or other one-celled organisms.

mineral The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together.

NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

paleobiology The study of organisms that lived in ancient times — especially geologically ancient periods, such as the dinosaur era. Scientists who work in this field are known as paleobiologists.

propulsion The act or process of driving something forward, using a force. For instance, jet engines are one type of propulsion for keeping airplanes aloft.

remnant Something that is leftover — from another piece of something, from another time or even some features from an earlier species.

scenario An imagined situation of how events or conditions might play out.

sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

stromatolite A type of layered rock that forms when cyanobacteria in water create huge communities. Their sticky surfaces trap sediments floating in the water. That accumulating sediment reacts with calcium carbonate in the water. This creates limestone, which builds up very, very slowly. A century-old stromatolite may grow a mere 5 centimeters (2 inches) in size.

unique Something that is unlike anything else; the only one of its kind.